When a mob dies in water, it usually slowly sinks to the bottom and that applies to every mob except a few. I know this is an intended feature because even aquatic mobs like fish or guardians sink to the bottom of the ocean when killed.

Currently to my knowledge, the only mobs that don't sink are axolotls, frogs, squids, glow squids, and striders. Of these mobs, the strider doesn't fall at all as listed in a previous bug reported by me, MC-255096, and squids and glow squids seem like mobs that would be an exception to this. This just leads the frogs and axolotls and I think this has something to do with them both able to swim through the water and this conflicts with this feature. I know it isn't a forgotten feature or anything because the tadpole was a recently added aquatic mob that also sinks in water during its death animation.

To replicate this behavior, find or create a body of water and use one of the following commands to summon one of the mobs listed above in the water.

/summon squid ~ ~ ~ {Health:0b,DeathTime:-200s}

/summon glow_squid ~ ~ ~ {Health:0b,DeathTime:-200s}

/summon axolotl ~ ~ ~ {Health:0b,DeathTime:-200s}

/summon frog ~ ~ ~ {Health:0b,DeathTime:-200s}

/summon strider ~ ~ ~ {Health:0b,DeathTime:-200s}

You can compare this behavior to another aquatic mob by using one of the following commands below

/summon cod ~ ~ ~ {Health:0b,DeathTime:-200s}

/summon dolphin ~ ~ ~ {Health:0b,DeathTime:-200s}

/summon guardian ~ ~ ~ {Health:0b,DeathTime:-200s}

You will see that the prior listed mobs will sink and any other mob like the cod, dolphin, and guardian will sink to the bottom.